A light-emitting element having a structure in which an organic compound that is a light-emitting substance is provided between a pair of electrodes (also referred to as an organic EL element) has characteristics such as thinness, light weight, high-speed response, and low voltage driving, and a display including such a light-emitting element has attracted attention as a next-generation flat panel display. When a voltage is applied to this light-emitting element, electrons and holes injected from the electrodes recombine to put the light-emitting substance into an excited state, and then light is emitted in returning from the excited state to the ground state. The excited state can be a singlet excited state (S*) and a triplet excited state (T*). Light emission from a singlet excited state is referred to as fluorescence, and light emission from a triplet excited state is referred to as phosphorescence. The statistical generation ratio thereof in the light-emitting element is considered to be S*:T*=1:3.
As the above light-emitting substance, a compound capable of converting singlet excitation energy into light emission is called a fluorescent compound (fluorescent material), and a compound capable of converting triplet excitation energy into light emission is called a phosphorescent compound (phosphorescent material).
Accordingly, on the basis of the above generation ratio, the internal quantum efficiency (the ratio of the number of generated photons to the number of injected carriers) of a light-emitting element including a fluorescent material is thought to have a theoretical limit of 25%, while the internal quantum efficiency of a light-emitting element including a phosphorescent material is thought to have a theoretical limit of 75%.
In other words, a light-emitting element including a phosphorescent material has higher efficiency than a light-emitting element including a fluorescent material. Thus, various kinds of phosphorescent materials have been actively developed in recent years. An organometallic complex that contains iridium or the like as a central metal is particularly attracting attention because of its high phosphorescence quantum yield (see Patent Document 1, for example).